I did this because I wanted a non-metal (shortest possible) mag for hunting. However I now needed to limit the mags capacity (because they hold 7) to it's legal hunting capacity (in Maine) to 5 rds total.
Magpul has recently release new mag limiters for their 20rd PMags. They come in a 5 & 10 rd variety. For the .450, the 10rd limiter is what we are going to use http://store.magpul.com/product/MAG281/101
Pre Mod Limiter:
- Limiter
- 10_rd_limiter.png (95.01 KiB) Viewed 12083 times
As seen in these pics (not mine, but used/modded for demonstration purposes). If you cut the top off the limiter, the 20rd will now hold a max of 5 .450 rds and still allow the use of the dust cover. The best part is this limiter is made for the PMag system so it works flawlessly.
Here is the place you need to cut:
- With cut line
- 10_rd_limiter_with_cutline.png (93.56 KiB) Viewed 12083 times
What the finished limiter will look like for a max capacity of 5 .450 rds in the mag:
- cut
- 10_rd_limiter_with_cut.png (78.61 KiB) Viewed 12083 times
Very simple and easier then making a custom block.
I've never seen a Pmag and thought I was looking at the follower, not some device that fits in the bottom, just above the floorplate. OK, here's my Emily Latella voice. 
